On page 398 of my Speer Reloading Manual number 11 Fourth Printing, March 1991. It says SAAMI working pressure for .38spl is 18,900 cup and +P 22,400 cup. The new numbers are 17,000 and +P 20,000. That is what happened to .38spl carry ammo.

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dd884
JMHO-YMMV

You are confusing CUP with PSI. The 1991 numbers you quoted are in a different testing media than the newer numbers. The newer Piezo testing method reported in PSI (pounds per square inch) is a more accurate method of testing. There is no way to reliability convert the two.

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Freedom is never free!!
SWCA #3437

The two most recently published SAAMI Manuals show exactly the same figures for both standard pressure (17,000 PSI by Piezo) and +P (20,000 PSI by Piezo). If you look at old loading manuals that presume to publish SAAMI pressures at that time and you will find they are all over the park!

First, go to the original source, not what a secondary reference says. Especially if it is Speer! The figures you quote from the Speer are incorrect! At that time the correct figures were 15,000 CUP and 18,000 CUP measured by Copper Crusher means.
And, what difference does it make really, you do not have the means of measuring pressure anyway, you are at the mercy of the manual publishers to publish data that is within SAAMI limits!

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Gunsmithing since 1961

In 1993, the pressure data published by SAAMI (ANSI/SAAMI Z299.3-1993) for .38 Special and .38 Special +P were as follows:

.38 Special:
Page 12 - Maximum Average Pressure (CUP) - 17,000
Page 16 - Maximum Average Pressure (PSI) - 17,000

.38 Special +P:
Page 12 - Maximum Average Pressure (CUP) - 20,000
Page 16 - Maximum Average Pressure (PSI) - 20,000

In 2015, the published data from SAAMI (ANSI/SAAMI Z299.3-2015) was identical to that published in 1993:

.38 Special:
Page 12 - Maximum Average Pressure (CUP) - 17,000
Page 20 - Maximum Average Pressure (PSI) - 17,000

.38 Special +P:
Page 12 - Maximum Average Pressure (CUP) - 20,000
Page 20 - Maximum Average Pressure (PSI) - 20,000

While everyone seems to agree that CUP and PSI cannot be accurately converted one to the other, it would appear that SAAMI hasn't yet gotten that memo.

The 1993 standard is, evidently, no longer available online. However, ANSI/SAAMI Z299.3-2015 is viewable and downloadable (as a PDF), in its entirety, here:
SAAMI

If anyone is interested, I have the 1993 SAAMI standard, ANSI/SAAMI Z299.3-1993, saved as a PDF and will gladly share it.

Yes I know, but what was it in 1985, before it was lowered in @ 1991 or 2?

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dd884
JMHO-YMMV

"While everyone seems to agree that CUP and PSI cannot be accurately converted one to the other, it would appear that SAAMI hasn't yet gotten that memo."

Possibly SAAMI doesn't worry much about CUP because I doubt that any ammunition or powder manufacturer in the civilized world today still uses copper crushers for pressure measurement. A comparison I have made previously is that using the copper crusher method to measure pressure is sort of like telling time with a sundial or hourglass. The copper crusher method is slow, very labor intensive, imprecise, and doesn't actually measure pressure at all, but rather something which is roughly analogous to pressure. I have no idea when the SAAMI pressure limits changed, but I would bet sometime in the late 1960s-early 1970s when piezo gauges first appeared on the scene for use in chamber pressure measurement.

From what I have seen, most of Hodgdon's data still list CUP pressures. Many show PSI, but most are CUP. If a major powder and/or bullet manufacturer lists load data, you can probably feel safe using it. I always recheck other sources, for same bullet/powder data, and they usually overlap somewhat. But loads shown in CUP from 25 years ago, which now show up in PSI for the same charge weight, are as safe as they always were. There is a guy on another forum that said "Hodgdon's data is not safe", because it shows CUP. His exact quote! He is an industry professional...a retired, supposed reputable gunsmith. There were a few others besides myself, that refuted that comment. If those charges from way back then, didn't blow up any guns, why would they now?
He posts here sometimes, with a slightly different name. He may chime in...

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The CUP measurement understates actual peak chamber pressure measured using a piezo gauge by 15%-25% depending upon many factors. You can think of the CUP as being an approximate "average" pressure due to its very long time constant (response time). On the other hand, piezo gauges have a time constant of essentially zero. CUP is an obsolete technology best left in the history books than depended upon for any purpose today.

While CUP measurements may understate pressures, if they are used as a gauge of where to draw the line, that is not an issue. The problem comes when you try and use them interchangeably. I understand that the newer technology is superior, but the old technology was effective within it's limitations.

I dont sweat the details between the two enough to eliminate one method over the other.

shooting the PSI calibrated load ... hey, no problem.
trying the old CUP data ... still no problem.
Why? ... odds are the gun you will be firing either with was designed around the time CUP was the gold standard While the newer PSI standards seem to impose some margin.
Besides ... either standard still falls short of a blue pill proofing load.

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it just needs more voltage